You need to use the setPos command.
If your parachute vehicle is named "chute", then your code could look like the following in a script:
_pos = getPos chute
chute setPos [(_pos select 0), (_pos select 1), (_pos select 2) + x]
OR
chute setPos [(_pos select 0), (_pos select 1), x]
Replace x with some value.
The first method will add (or subtract) from the parachute's current altitude. The second method will set the parachute's altitude directly to x.
With a little work, you can use this code in a trigger's or waypoint's On Activation field. For example:
chute setPos [(getPos chute) select 0, (getPos chute) select 1, x]
Good luck.